Parallel Adatom Chains on Si(111): A Chemisorption-Induced Surface Reconstruction

Using surface x-ray scattering, we have studied a (3 ×{} 1) Si(111) reconstruction induced by chemisorption of CaF₂. This reconstruction consists of a parallel array of zigzag Si adatom chains oriented along 1{}10>. The orbitals on the Si chain atoms rehybridize into pₓ and py along the surface to form σ bonds, and parallel spz orbitals perpendicular to the surface to give additional π bonding. This basic structure, with no dangling bonds at only ⅓ monolayer adsorbate coverage, may explain the occurrence of (3 ×{} 1) reconstructions among a diverse range of Si-adsorbate systems.